Communication systems are used to transfer information from one location to another. The content and format of this information can vary greatly depending upon the type of system and the application. For example, there is a great need to communicate digital information such as data, voice, video and others. Depending upon the channel used, this information is often transmitted in analog form.
FIG. 1 illustrates a simple block diagram of a conventional receiver 10. An analog signal is received at analog receiver 12. The signal is then digitized in analog-to-digital converter (ADC) 14. The digital information can then be processed using a digital processor 16.
As is well known, the Nyquist theory states the minimum sampling rate required to turn an analog signal into an accurate digital representation. Specifically, the sampling rate must be at least twice that of the highest component of the analog frequency in order to accurately reproduce the sampled signal. FIGS. 2a-2c illustrate this point.
FIG. 2a shows the frequency spectrum of an arbitrary analog signal. As shown in FIG. 2b, when the signal is sampled, an image spectrum will be generated. The image spectrum will be a mirror image of the original spectrum with the sample frequency serving as the axis of symmetry. This result leads to the Nyquist theory. If the sample frequency is greater than the maximum frequency of the original spectrum, the original spectrum will be retrievable. If, however, the sample frequency is less than the maximum frequency, the image spectrum will overlap the original spectrum, as shown in FIG. 2c. This effect is known as aliasing. The portion of the image spectrum that overlaps the original spectrum, referred to as the aliasing spectrum or alias band herein, will distort the original spectrum and prevent accurate reproduction.